Billy Hamilton hit a walk-off single to complete the Reds biggest comeback of the season and beat the Cubs 6-5.
After falling behind 5-0 in the early innings, the Reds rallied to tie the game in the eighth inning on a Jay Bruce double and Hamilton won it with his second hit of the game.
"This is why we're going to win because of the heart we have," said Hamilton. "When we're down, we just keep battling the whole time. This team is great, (it's) a great win."
David Holmberg, making his Reds debut, allowed all five runs before the end of the third inning. The Reds bullpen shut the Cubs out the rest of the game.
Every Reds starter had a hit and the Reds completed their first doubleheader sweep since 2009.
The Reds have won the first three game of the five game series with Chicago. Same two teams play Wednesday night at Great American Ball Park.
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